1. Field of the Invention
This invention relates to solar collectors and more particularly to a new and improved solar collector having a selective film of improved stability to liquid water.
2. Description of the Prior Art
It is well recognized that the fossil fuels of the world are being depleted. Indeed in certain oil-producing countries, the known reserves of oil are of quite limited magnitude. It would therefore appear of a high level of importance to utilize the energy of the sun, i.e. solar energy, to the extent we feasibly can to meet our energy requirements. Solar energy could, in theory, supply all of the energy requirements of this country.
Considerable research and experimentation has been carried out heretofore in the field of solar thermal energy conversion, and the technology of solar thermal energy conversion is fairly well advanced. One of the most important of a number of fairly recent discoveries in this field that have significantly improved the efficiency of solar thermal energy collectors is the use of "selective" surfaces, which are also referred to as spectrally selective surfaces. A surface that is both a good absorber of solar radiation and a poor emitter of infrared radiation is said to be "selective" or spectrally selective. A selective surface can significantly improve the thermal efficiency of solar thermal energy collectors or solar heat collectors. It is essential that the selective surface be stable or durable under the conditions encountered in the operation of solar heat collector.
Prior art selective surfaces suffer from a stability or durability problem, in that liquid water contacting the selective surfaces of the solar collectors when the collector box is not sealed, such as the water formed by the condensation of moisture from the air within the collector box in the cooler temperatures of evening, detrimentally changes the thickness of the selective copper oxide coating on the copper panel or flat plate surface resulting in an undesirable, considerable reduction in the selectivity of the black coating. Consequently, the solar absorptivity of the selective coating of the collector is considerably lowered and usually to an undesirable value below 0.90 and the thermal emissivity of the selective coating is considerably increased and usually to an undesirable value above 0.20. Such lack of desired stability of the selective coating to contact with water is especially a problem, when the selective coating or film is a black copper oxide (CuO) coating or film on a copper or copper alloy substrate and is produced by contacting the substrate with a prior art aqueous blackening solution comprising an oxidizing agent for the copper such as an alkali metal chlorite, and caustic soda.
U.S. Pat. Nos. 2,364,993 and 2,460,896 disclose the deposition of black coatings or films on copper and copper alloy surfaces by immersing the surface in a bath comprising an aqueous solution of alkali metal hydroxide, e.g. caustic soda, and sodium chlorite or potassium chlorite. U.S. Pat. No. 2,481,854 also discloses the blackening of surfaces of copper and coppeer alloy by contacting the surface with an aqueous solution comprising sodium chlorite or potassium chlorite, an alkali metal hydroxide, e.g. sodium hydroxide. The chlorite utilized in preparing the blackening solution is uniformly fixed in the water of hydration of the alkali metal hydroxide. The publication entitled "Spectral and Directional Thermal Radiation Characteristics of Selective Surfaces for Solar Collectors" by D. K. Edwards, J. T. Gier, K. E. Nelson and R. D. Roddich, presented to the United Nations Conference on New Sources of Energy, Apr. 20, 1961, discloses that promising low temperature collector surfaces appear to be copper or steel treated by the commercial "EBONOL" processes. This last-mentioned publication also discloses that commercial chemical dip treatments of copper and steel are shown to give selective characteristics desirable for low temperature collectors such as solar water heaters, and that a copper surface dipped 5 minutes in "EBONOL" C blackener solution at 175.degree. F. produced a coating on the surface having a solar absorptivity of 0.91 and a thermal emissivity of 0.16 for collectors in space.
Prior art solar energy collectors and heaters are disclosed by U.S. Pat. Nos. 1,425,174; 1,888,620; 1,889,238; 1,971,242; 2,202,756; 2,208,789; 2,931,578; 1,034,475; 2,553,307; 3,176,678 and 629,122.